Known particle counters operating according to the opacity method are very precise and costly laboratory devices. They include a high quality precision optic, which contributes to the high level of precision and makes these devices very sensitive to outside negative influences, especially vibrations or jarring, air humidity and temperature changes. The high cost results from costly operation and a costly electronic analysis device. Accordingly, these known particle counters are not even considered for use in industry, for instance in a hydraulic system.